1. Field of the Invention
The present invention relates to a data communications apparatus for controlling a retransmission.
2. Description of the Related Art
Normally, high reliability to assure that data surely reaches a transmission destination is required when a data communication is made. For example, in a data communication made on the Internet, TCP (Transmission Control Protocol) is used as a standard protocol. This is because TCP is a protocol having a retransmission capability.
For instance, in a data communication using TCP, a transmitting side sets a retransmission timer each time it transmits transmission data (transmission segment). Upon detection of a timeout, the transmitting side retransmits the same transmission data. In the meantime, a receiving side checks a sequence number attached to the header of the transmission data, and issues a request to retransmit the transmission data that does not reach.
FIG. 1 is a block diagram exemplifying a configuration of a conventional data communications system.
An opposed communications apparatus 1201 shown in FIG. 1 is configured with an application processing unit 1201a for making a data communication, a protocol processing unit 1201b for enabling a communication between arbitrary computers and for assuring transmission data, and an interface 1201c for converting an electric signal on a network into digital data and for enabling a data transmission between the computers.
For example, the application processing unit 1201a is equivalent to an application layer of TCP/IP (Internet Protocol), and the protocol processing unit 1201b is equivalent to TCP and IP layers of TCP/IP. The interface 1201c is equivalent to a network interface layer of TCP/IP.
Also a communications apparatus 1202 is configured with an application processing unit 1202a, a protocol processing unit 1202b, and an interface 1202c. The opposed communications apparatus 1201 and the communications apparatus 1202 are interconnected via a communications network 1203 to make a communication. The communications network 1203 may be directly connected, or may be connected by a network via a plurality of apparatuses.
FIG. 2 is a schematic showing the outline of a communication process performed in the conventional data communications system. A case where the opposed communications apparatus 1201 and the communications apparatus 1202 make a data communication is described below in correspondence with (1) through (14) shown in FIG. 2.
(1) When the opposed communications apparatus 1201 starts a communication, the application processing unit 1201a transmits transmission data to the protocol processing unit 1201b. 
(2) The protocol processing unit 1201b, which receives the transmission data, transmits a connection request to the communications apparatus 1202, being a transmission destination, in order to establish a connection. At this time, the connection request is transmitted to the communications apparatus 1202 via the interface 1201c and the communications network 1203, although this is not shown in FIG. 2.
(3) When the communications apparatus 1202 accepts the connection request signal, the protocol processing unit 1202b transmits a connection response to the opposed communications apparatus 1201, being the transmitter, and establishes a connection.
(4) When the connection is established, the protocol processing unit 1201b partitions the data received from the application processing unit 1201a into a predetermined size, and starts to sequentially transmit the partitioned data to the communications apparatus 1202.
(5) In the meantime, in the communications apparatus 1202, the protocol processing unit 1202b transmits a data reception response to the opposed communications apparatus 1201 each time the data is received.
(6) When the reception of the communication data is complete, the protocol processing unit 1202b restores the transmission data partitioned in (4), and transmits the restored data to the application processing unit 1202a. 
The above described process is repeated, whereby the transmission process of the data from the opposed communications apparatus 1201 to the communications apparatus 1202 is performed. Also a case where the communications apparatus 1202 makes a data transmission to the opposed communications apparatus 1201 is similar.
Here, a case where the opposed communications apparatus 1201 transmits data α and β to the communications apparatus 1202 is considered.
(7) The protocol processing unit 1201b, which accepts the data α from the application processing unit 1201a, transmits the data α to the communications apparatus 1202.
(8) The protocol processing unit 1202b, which receives the data α, transmits a reception response to the data α to the opposed communications apparatus 1201 which is a transmitter.
(9) Furthermore, the protocol processing unit 1202b transmits the received data α to the application processing unit 1202a. 
(10) Similarly, the protocol processing unit 1201b, which accepts the data β from the application processing unit 1201a, transmits the data β to the communications apparatus 1202.
(11) For example, if collision of data occurs on the communications network 1203, the data β transmitted from the opposed communications apparatus 1201 disappears and is not transmitted to the communications apparatus 1202.
(12) In the meantime, the protocol processing unit 1201b transmits the data β, and activates a retransmission timer to monitor the reception response to the data β. Upon detection of a timeout, the protocol processing unit 1201b retransmits the data β.
(13) The protocol processing unit 1202b, which receives the data β, transmits the reception response to the data β to the opposed communication apparatus 1201 which is the transmitter.
(14) Furthermore, the protocol processing unit 1202b transmits the received data β to the application processing unit 1202a. 
At this time, for example, if memory for storing the data β cannot be secured for a reason that the throughput of the application processing unit 1202a is inferior to that of the protocol processing unit 1202b, the data β is destroyed and the application processing unit 1202a cannot receive the data β.
Namely, the application processing unit 1202a has completed the reception of only the data α although the application processing unit 1201a has completed the transmission of the data α and β to the communications apparatus 1202.
Accordingly, the conventional retransmission process shown in (10) through (12) cannot cope with the case where transmission data disappears between the protocol processing unit and the application processing unit as shown in (12) through (14), leading to a problem that the reliability of the data is degraded.
Japanese Published Unexamined Patent Application No. H11-177536 discloses a wireless data link layer error control method for preventing a throughput characteristic from degrading due to the number of unnecessary retransmissions, which grows with an increase in transmission errors caused by degradation in a line state, by monitoring the line state and suspending a retransmission control when the line state degrades below a reference value, and by restarting the retransmission control when a monitoring result restores to a more favorable value than the reference value.
Additionally, WO Patent Publication No. 2002/056631 discloses a mobile communications system, which can reduce a delay time required for a retransmission between two processing units of a layer by suppressing the missing of data and a retransmission request frame for a retransmission request between the two processing units of the layer in a base station and a mobile station, and can prevent a throughput from degrading by suppressing an occurrence of a timeout in an upper TCP.